Image projection systems may be used to display a still or video image, or to enable the image to be viewed simultaneously by a large or small audience. Although such projection systems are intended to produce image color and brightness as faithfully as possible, such fidelity may be limited by, among other factors, a light source having a less than ideal spectral power distribution.
Spectral power distribution is a measurement of the luminous flux of light emitted by a light source, as a function of the wavelength (and therefore the color) of the light. An ideal light source may exhibit a uniform emission intensity at each wavelength across the visible spectrum. Unfortunately, light sources typically offer non-ideal spectral power distributions, that is, emission intensity may vary as a function of wavelength. For example, some incandescent lamps may emit proportionally more lower frequency red and orange light than higher frequency blue and violet light. Similarly, some discharge lamps (such as fluorescent lamps, sodium vapor lamps, or mercury vapor lamps) may exhibit very narrow emission bands in the visible region. In some cases, the discharge lamp may emit significant radiation in the nonvisible ultraviolet region. Such lamps may incorporate rare earth phosphor coatings capable of absorbing high frequency emissions and re-emitting light at lower frequencies, such as in the red region. However, such lamps may still exhibit highly nonuniform spectral power distributions.
Attempts to compensate for non-ideal light sources may include the use of color filters to decrease projected blue and green wavelengths relative to red wavelengths. For example, where the projector incorporates a sequential R,G,B,W color wheel, a proportionally larger segment of the color wheel may be used to transmit red light, relative to the size of the green and blue color wheel segments, to compensate for weaker red wavelength emissions. However, such solutions typically serve to decrease the total amount of light capable of being projected by the light engine. In addition, such compensating mechanisms may increase the cost and weight of the projector system, and in the case of the rapidly spinning color wheel, contribute to the acoustic noise of the projector's operation.